1. Dawn of Plant Life On Earth
“And the Earth was without form and void”
THE quotation from the second sentence of the first chapter of Genesis tells us more in eight words than could very well be said in as many chapters. Not only have we biblical authority for this early absence of life on the earth, but all the accumulated knowledge of the ages points in the same direction. We have already seen that plants, because they can take inorganic substances from the earth and air and transform them into organic food, must in all probability have come on the earth before animals which, directly or indirectly, all rely upon plants for their food. Even those animals that eat only flesh devour other animals which depend upon plants for food. It may safely be repeated, then, that upon plants all animal life depends, and that, in the dim beginnings of things on the earth, it must have been some form of plants that were the first living things. Extremely simple unicellular animals, however, are known to date from early times.
In the volume devoted to that subject in this series, you will find that at the very earliest stages of what we know as our globe there was a segregation of land and water somewhat different from our great oceans and continents to-day in extent and area, but differing mostly in this—that much of the water was fresh and very nearly boiling hot. We have still the remnants of those great reservoirs of hot water, as our hot springs and shooting geysers only too well prove. And if all plants were as quickly killed by hot or boiling water as the common garden geranium, we should not expect any plant life to have developed upon the earth until all those great bodies of water had cooled. To have waited for that would have been to delay the appearance of plants for no one knows how many millions of years, and there is some fairly good evidence that long before normal conditions of heat and cold were established there already flourished certain kinds of plants. What those plants were is something of a speculation, and indeed exactly what they were no one knows. But in our present hot springs grow certain plants, microscopic in size, but quite obviously related to the algæ or to certain bacterialike organisms. They live with apparent comfort and reproduce themselves freely in water so hot that no other form of life will maintain itself. While there is no proof that these present plant inhabitants of hot springs, common in the West, are descended from infinitely ancient progenitors, it is a fair assumption that some organism capable of growing in warm or hot water was the first living thing to appear in a world otherwise “without form and void.”
This great question of how plants came on the earth, and particularly how from these apparently simplest organisms our whole wonderful vegetation has arisen, has always been one of the most interesting things in the history of the world. There are many different ways of studying this, and in the very earliest stages of plant development we are forced to reason, not so much by actual records or buried skeletons of the plants that probably existed then, for only a very few have ever been found, but by our knowledge of the physical and chemical requirements of unicellular plants, and those slightly more developed, and of their individual life histories. It is, for instance, certain that the first plants must have been aquatic, as no real land plants are known for hundreds of thousands of years after the earth was quite capable of maintaining plant life. The absolute necessity of water to complete fertilization in nearly all cryptogams also makes it fairly certain that water plants, and these of the simplest nonflowering type, were the first living things to be found on the earth. And it is more than a fair inference that these were inhabitants of warm or hot water. Subsequently, as the water cooled, they may well have been not unlike the green scum found on ponds to-day.
Of course, the actual origin of life itself is still as much of a riddle as it was when the ancient philosophers began to speculate about it years before the Christian era. Protoplasm, the unit or basis of all life, while its composition and growth requirements are fairly well known, has never been made in any laboratory. Nor have scientists ever been able to decide what the combination of physical and chemical forces must have been to originate it. But that from a perfectly sterile, probably steaming hot globe, there did finally develop some form of life, and that this must have been aquatic plant rather than animal life, seems not only certain, but the only hypothesis upon which all subsequent development of life must have been based. It is not necessary to ascribe the origin of life to providential inspiration nor to the meddling of strange and outlandish deities, as all savage tribes did and some more civilized peoples still do. There can, however, be no escaping the fact that life is more than the combination of physical and chemical conditions which sustain it, and that its origin has never, and may never be “explained” by merely describing the conditions which unquestionably favored its appearance. In other words, the origin of plant life throws us back upon things believed but incapable of proof, and is none the less wonderful because we cannot yet understand the probable progression of forces and materials to which it owes its origin.
Assuming, then, and we must all start with this proposition, that aquatic plants certainly, and warm or hot water plants probably, were the first living things upon the earth, what are the next steps in the history of the plant kingdom? The answer to that question involves a few simple facts in geology and, particularly, in the making of fossils, which must be understood before we can see those steps or their significance. The geological changes which have resulted in the present condition of the earth’s surface are described in the volume devoted to that subject, and will not be repeated here. But some mention must be made of the formation of fossil plants, particularly as it is upon the evidence of these that the story of the development of plant life is literally written in the rocks.
If a leaf or twig drops into shallow water with a clay or mud bottom, it ultimately sinks, and if then a film of clay or silt is brought down by freshets or what not, it will bury the leaf or twig, of course, filling in every slight depression. If then the buried object were raised so that it dried out and could be split open, we should find a perfect impression of the veins and other outward characters of the leaf etched in the clay. This is often such a perfect process that every detail of the leaf is left in the mud impression, and only the opportunity for this impression to become hardened into rock is needed for us to have a fossil. For these are merely the final hardened rock stages of a process that began as we have indicated, and the thousands of fossils that have been dug out of the earth prove how common the conditions for their formation must have been in certain periods of the world’s history.
But of the untold millions of fossils that have been made most have been destroyed, for the geologists tell us that the earth’s crust has been subjected to much upheaval. Mountain chains thrown up, inland seas formed, great river systems carved out, and tremendous periods of vulcanism or fire action have made the earth’s crust, at different periods, a mighty restless place. And these changes, so slow that often millions of years have elapsed before they were completed, have sometimes been favorable to the making of fossils and sometimes to their destruction. Darwin once wrote about the fossil record that he saw it “as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines. Each word of the slowly changing language, more or less different in the successive chapters, may represent the forms of life, which are entombed in our consecutive formations, and which falsely appear to have been abruptly introduced.” And yet it is upon the evidence of this fossil record that most of our knowledge of the history of the plant kingdom is based.
The difficulty of getting any true picture of the beginnings of plant life is great, for those earliest stages of development were unquestionably water-inhabiting plants, whose tissue is mostly too soft and too easily decayed to make fossil impressions of them likely to be preserved. Yet fossil algæ have been found in rock strata so old that no fern or flowering plant had yet made its appearance. It is not too much to picture the world then as peopled only by cryptogamous plants of simple structure, living in the water, and land plants which to-day make up the bulk of our vegetation as not yet developed. Furthermore, there is in these earliest stages no trace of plants with any kind of a vascular system, such as all ferns and flowering plants possess. No stretch of our imagination can readily picture the earth as it in all probability was in that period, with no trees or vines or flowers, the land wholly bare of vegetation, and in the water, along its sterile shores, only unicellular or slightly more developed, wholly nonflowering plants. The conditions supporting such plant life existed for many millions of years, and some geologists have claimed that this period of time exceeded all the subsequent ones combined, so that algæ and some other unicellular plant types are the oldest in the world; and they still exist in enormous numbers.
Much later than this, fossil algæ of comparatively complex structure have been found, showing by their frequency and more highly developed characters a more advanced stage in the development of the plant kingdom. So common were these various types of what we now call seaweeds, although most of them apparently lived in fresh water, and so widespread was their occurrence, that this Pre-Cambrian period has often been called the reign of algæ. As yet no other plants had been developed and none of these ancient types had invaded the land, which for millions of years more must have been entirely without vegetation.